If you have changed your core files then you are best off not upgrading.
I am not sure what I should do because last time, I lost everything.
Suffusion offers you several upgrade-proof ways even if you are not using a child theme. E.g. all CSS changes can be included in Back-End Settings → Custom Includes → Custom Styles. Changes made there are preserved through theme upgrades. There are quite a few articles that provide instructions on how to use this feature. Most PHP changes can be handled via child themes. And everything changed via the back-end "Suffusion Options" is seamlessly ported over without any additional steps.
What kind of thing could I do before I update?
As you have made PHP and CSS file changes, your only option is to back them all up, then merge them with the new version (that is a non-trivial task if you haven't kept track of what you have changed).
Do I even need to update? I am guessing yes.
There is a full change-log published with each release of the theme: http://aquoid.com/news/category/theme-releases/. Feel free to evaluate the listed changes and see if you need any of them. If you feel that any of them is required, then upgrade. Otherwise if you are on a version that works well for you (and you have changed files on it to the point of no return), you should be fine without upgrading.
voodoohoodoo
Di Smith
jenagan